Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrier confinement

We have used low energy electron-excited nanoscale luminescence spectroscop y (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device struc tures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement, We investigated AlGaN/GaN heterostructures with different ele ctrical properties using incident electron beam energies of 0.5 to 15 keV t o probe electronic state transitions within each of the heterostructure lay ers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentra tion grown on GaN buffer layers on sapphire substrates by plasma-assisted m olecular beam epitaxy exhibited a range of polarization-induced electron de nsities and room temperature mobilities. In general, the spectra exhibit Al GaN band edge emission at similar to3.8 eV or similar to4.0 eV, GaN band ed ge emission at similar to3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV and a large emission in the infrared (less than or equal to1 .6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. Th ese heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and with out 2-DEG confinement and highlight the importance of AlGaN defects in the near 2DEG region.